1. Sprinkle a hearty amount of baking soda on the drain area
2. Add a large amount of vinegar and let the bubbling reaction finish before moving on
3. Flush with very hot water and fill up the sink with water
4. Use the plunger to force air down the drain. At this point you may see some gunk come out the overflow hole near the top of the sink. Remove anything that comes out.
5. Remove the stopper in the drain and scrub all around inside with a toothbrush. You will likely pull out clumps of hair stuck to gummy pieces of black gook
6. Flush once more with vinegar, followed by water
Works every time (hey, I didn't say it was pretty, but it works!)